1. Field of the Invention
The present invention relates to a flue gas desulfurization process wherein sulfur oxides are separated by absorption from combustion waste gases.
2. Description of the Related Art
A number of flue gas desulfurization processes using a calcium compound as an absorbent, as typified by the wet lime process, have already been used in practice. For example, the wet lime-gypsum process used in many large-scale flue gas desulfurization systems for commercial power plants involves gas-liquid contact between flue gas containing sulfur oxides and an absorbent solution containing a Ca compound so as to remove the sulfur oxides by absorption and recover gypsum, also referred to as plaster as a by-product.
In the art Of flue gas desulfurization using a Ca compound as an absorbent, it is known that the addition of a water-soluble organic carboxylic acid together with the Ca compound improves the performance of the absorbent for the removal of sulfur oxides (hereinafter called the "desulfurization efficiency"). For instance, the specification of Japanese Patent Provisional Publication (KOKAI) No. 61-8115 reveals that adipic acid, which is an organic carboxylic acid, was found to enhance the desulfurization efficiency when added to the absorbent solution in a flue gas desulfurization apparatus. The enhancement of the desulfurization efficiency by an organic carboxylic acid, e.g., adipic acid, is presumably attributed to the mechanism which will be described in the following. Adipic acid in the absorbent solution is maintained in an equilibrium state as represented by the formulas (1) and (2). It incorporates hydrogen ions [H.sup.+ ] that result from the absorption of sulfur oxides, and attains a kind of buffer effect to prevent a pH drop, thus leading to an improvement in desulfurization efficiency. EQU HOOC(CH.sub.2).sub.4 COOH=HOOC(CH.sub.2).sub.4 COO.sup.- +H.sup.+ (equilibrium) (1) EQU HOOC(CH.sub.2).sub.4 COO.sup.- =OCC(CH.sub.2).sub.4 COO.sup.2- +H.sup.+ (equilibrium) (2)
On the other hand, the conventional flue gas desulfurization processes require a portion of the absorbent solution to be drawn out as waste water for such purposes as preventing the accumulation of impurities in the solution. While an organic carboxylic acid improves the desulfurization efficiency as noted above, it is soluble in water and therefore becomes entrained by the waste water. The waste water with the organic carboxylic acid entrained causes the following problems.
(1) As the Japanese Patent KOKAI No. 61-8115 points out, waste water containing an organic carboxylic acid may not be released into environment as it is. In such a case the organic carboxylic acid must be decomposed before disposal, and use becomes necessary of costly equipment such as one for the activated sludge processing.
(2) The organic carboxylic acid entrained by waste water is difficult to recover and becomes lost. Therefore, a fresh supply has to be added to the system in an amount equivalent to the loss. Since organic carboxylic acids are generally expensive, the replenishment can become a major economic burden.
(3) To the problems (1) and (2) above, the Japanese Patent KOKAI No. 61-8115 proposes a process wherein waste water (which is called "mother liquor" in this publication) is evaporated to dryness for the recovery of the organic carboxylic acid. The process concurrently recovers water-soluble salts (calcium chloride, magnesium chloride, etc.) that are principal components of waste water. If the organic carboxylic acid is to be reused, it must be separated from the recovered matter and purified before reuse. Although this publication is silent on how to do it, the separation and purification is considered to be very difficult because both water-soluble salts and organic carboxylic acids are soluble in water.